1. Field of Invention
This invention relates to adjustable guitar structures and their construction, specifically to provide a method to accurately intonate acoustic guitars.
2. Prior Art
Typically stringed instruments of this type are composed of a series of strings that are strung under tension over a fret board being supported by a nut and saddle or saddles of the bridge. The frets are affixed to the fret board and occupy intermediate positions between the nut and saddle and are used as temporary points of string support to obtain higher frequency tones. To utilize the frets the strings are displaced until contact occurs with the fret. In guitars proper tonal quality is obtained when the strings are tensioned and supported to produce the tonal harmonic at the 12th fret, i.e., correct intonation. To achieve this guitars are proportioned so that the distance to the 12th fret from the nut is approximately equal to the distance from the 12th fret to the saddles. In practice many factors enter into gaining the location to produce the harmonic. These factors fall into several general categories; those governing the vibration of the strings, and those involved in transmitting the vibrational energy to the guitar's top (sound board). The factors governing string frequency is: tension, active string length, and linear density of the string. Transmission of vibrational energy is affected by the material and mechanical composition of the bridge and saddle(s).
Conventional guitar construction utilizes a straight saddle positioned to produce correct intonation for the high and low E strings using what the manufacturer considers to be an “average string.” The remaining strings seldom if ever achieve the precise degree of intonation that produces optimal sound characteristics. Moreover, a multitude of string types are available, each type possessing differing material compositions and gauges producing unique linear densities and tension requirements. Players possessing refined musical abilities generally have strong personal preferences as to how their instruments sound and play. To achieve these preferences they frequently select strings that are different than those the instrument comes outfitted with and adjust the action, i.e., the string height above the fret board to complement their playing style. Thus, it becomes problematic whether even the high and low E strings will have good intonation.
In the case of professional artists who play in studio settings and groups, precise intonation becomes crucial to the production of high quality performance. Moreover, these musicians frequently travel to perform in different climatic zones which causes their instruments to de tune and require adjustment of the intonation. Furthermore, airplane travel further exacerbates these problems because of the rapid changes in pressure and altitude. Therefore, individual adjustability of string intonation is a desirable feature to provide a means to compensate for all the factors that degrade the sound quality of acoustic guitars. Thus, a need exists for an improved construction of an adjustable intonation apparatus to properly intonate acoustic guitars. The adjustable acoustic guitar bridge claimed here in provides a means to precisely and easily adjust individual string intonation without negatively affecting the sonority of the instrument.
Attempts have been made to produce a device to properly intonate acoustic guitars, none of which has proven to have any significant degree of commercial viability. In the 1960's attempts were made by Gibson® with the Dove® acoustic guitar by putting their Nashville Tune-o-Matic® electric guitar bridge on an acoustic instrument without success. While adjustable electric guitar bridges theoretically allow an acoustic guitar to be properly intonated, typically the construction employed relies on an adjustment screw running through the metal saddle, with the screw connected at both ends, or a screw loaded with a spring between the saddle and screw to help stabilize the saddle. The type of construction described above is not adaptable to acoustic guitars because the transmission of string vibration is dampened resulting in diminished sustain (sound duration), sonority, and amplitude. Moreover, the metal parts in electric guitar bridges used to support the string are not acoustically resonant producing an undesirable thin brittle tone.
In the 1970's a compensated acoustic guitar bridge was developed which divided the slanted saddle into multiple segments that would provide a finer fit to individual string length requirements for better intonation. The most effective variety of this system of compensation utilized an individual saddle for each string that occupied fixed positions within the bridge structure. The location for each saddle position was derived for a particular string type selected by the player and was set up under “average” environmental conditions, i.e., using an environment controlled for humidity and temperature. Since this style of bridge is not adjustable it has many of the draw backs listed above.
Thereafter, inventors created a number of adjustable bridges for acoustic guitars. U.S. Pat. Nos. 6,124,536 to Hoshino (2000); 3,290,980 to Fender (1966) show bridge systems with saddles that are freely adjustable along the longitudinal axis of the strings. Adjustment of the saddles was accomplished by utilizing screws, the heads of which are restrained in position by passing through a hole located in a traverse boss on the posterior portion of the bridge or saddle mechanism with a similar boss that is mounted to a bridge. Typically the male threads of the screws engage sales that contain a threaded hole, allowing fine adjustment of the saddles location by rotating the screw. Free movement requires the various parts of the saddle traverse mechanism to incorporate clearances (spaces between the parts) to function. The clearances between the parts not only allows movement of the parts in the intended manner for string length adjustment, but also an undesired movement imparted by the vibration of the string. The undesirable vibration of the parts originating from string vibration uses sting energy, thereby, reducing the available energy for sound generation, resulting in a diminished sound amplitude potential for the instrument.
Cipriani, in U.S. Pat. No. 4,911,055 (1990) describes an adjustable bridge system with individually moveable saddles that incorporate a toothed bottom that engage a mating toothed surface incorporated upon the upper surface of a bridge base plate. The saddles are adjusted by moving them by hand to the nearest set of teeth to the desired saddle position on the corresponding bridge base plate, then passing the string over the saddle and utilizing its tension to maintain the saddle in the selected position. In this case the saddle position is incrementally adjustable and does not provide the degree of precision obtainable with a system that is continuously adjustable. Moreover, a significant probability would exist that the saddles could be dislodged from their positions when replacing worn out strings, which requires the removal of old strings and replacing them with new strings.
Widowson, in U.S. Pat. No. 2,491,788 (1949) depicts an adjustable bridge saddle system that incorporates a bridge, saddle platform, and longitudinally moveable saddles that are retained in position by a system of clamps. This system relies on the saddles to simply be moved into position by hand. While simple it does not provide the means of easily and precisely moving the saddle an exact amount as is possible with a system that is actuated by a screw.
More recently Yarosh published a design in the Journal of American Lutherie Number 82/Summer 2005 derived from Rodriguez's system as described in his book The Art and Craft of Making Classical Guitars (1960) which utilizes individual saddles mounted on wood blocks retained in lateral position by mating inwardly slanting slots (dovetailed slots) in the wood bridge. These slots are elongated parallel to the direction of the strings long axis allowing longitudinal adjustment. Adjustment of the saddle position is achieved by sliding the moveable saddle blocks in the elongated slots to the desired location and relying on string tension to maintain their position. Yarosh's design works in the same manner as Rodriguez's design but uses slots with vertical walls instead of the slanted dovetail, relying on the downward pressure of the string to retain the saddles in the bridge slots as well as their longitudinal position. Environmental factors, particularly humidity have significant effects upon wood, greater humidity causing expansion and lower humidity contraction. Thus, climate and weather conditions which affect humidity would cause variation in the size of the slots that retain the saddles. Thus, the wood expansion caused by elevated humidity would produce a reduction in the width of the saddle retention grooves, causing the saddles to be firmly gripped making movement for adjustment of saddle position difficult or impossible. Conversely, a lower level of humidity would cause shrinkage of the wood producing excessive clearance between the slots and the saddles. This would allow the saddles to become loose enabling the saddles to change position or even fall out during string changes. Moreover, under dry conditions the excessive saddle to slot clearance would allow the negative effects caused by string imparted saddle vibration as described above. A final problem with this design is the same difficulty of easily and precisely positioning the saddles as already described for Widowson's design.
The patents already mentioned, i.e., by Hoshino, Fender, and Widowson also have saddle geometries that can be characterized as having restricted string contact areas. This not a problem as long as the bridge unit is designed for instruments within a narrow range of string spacings. However, fretted string instruments are available in many neck widths producing differing angular string spreads that produce variation in the spacing of the strings when they pass over the saddles of the bridge. To accommodate string spread variation these designs would have to be produced in many size variants, making them from a manufacturing perspective less desirable than a system that is able to accommodate a wider range of string Spacings.